Digital watermarks (or simply, watermarks) are, most commonly, information or messages that are added to digital data, such as image, audio, video, document, and so forth, files. The information may typically be used to add copyright notices, security information, and so on, to the digital data, and may often be undetectable or practically undetectable. For example, the information may be used to denote the creator of an image or song. Alternatively, the information may be used as verification information to authenticate a file or to ensure that a file has not been altered (maintain file integrity).
A technique used to insert a digital watermark may involve spatially inserting the digital watermark into a file. For example, a creator's name inserted in a portion of an image or a digital watermark placed at some position of an audio file, such as, the beginning, middle, or end of the audio file. Another technique used to insert a digital watermark may involve inserting the digital watermark into a frequency domain representation of the file. Inserting a frequency domain representation of a digital watermark may result in a digital watermark that may both be difficult to detect and remove.
One disadvantage of the prior art technique using spatial watermarks is that in many cases, the spatial watermark can readily be removed, for example, by cropping or editing. When a watermark is easily removed, the security provided by the watermark is limited.
Another disadvantage of the prior art techniques is that simply inserting a watermark, either spatially or in the frequency domain, may result in a watermark that distorts the digital data, for example, visual distortion in an image file or audibly detectable distortion in an audio file.